Common industrial methods for producing various compounds containing vinyl functionality such as styrene, ethene, butadiene, isoprene, vinyl acetate, (meth)acrylic acid, (meth)acrylates, acrolein, acrylonitrile or vinyl-substituted aromatics, typically include separation and purification processes such as distillation to remove unwanted impurities or byproducts. However, undesired polymerization, especially during monomer purification processes such as distillation, results in loss of the monomer product. Moreover, loss of production due to polymer formation on process equipment continues to cause operating problems for those in the industry. In particular, plugging of distillation column overhead piping and fouling or plugging of condensers has been problematic. Therefore, the industry has sought compositions and methods that are less dangerous to handle, that are effective in multiple phases, and that reduce product losses and production problems.
Consequently, additives, which are referred to either as polymerization inhibitors or as polymerization retarders, are added to the olefinically unsaturated monomers generally as early as during the preparation process. Polymerization inhibitors are, as the name actually states, capable of effectively preventing undesired polymerization. Since the reaction rate of polymerization inhibitors is fast, polymerization inhibitors are consumed within a short time. The polymer content rises quickly thereafter. Polymerization retarders, in contrast, can partially prevent polymerization. The rate of polymerization slowly increases Therefore polymerization is effectively hindered or inhibited for a longer period of time, e.g. 4 hours. Due to slow reaction rate, polymerization retarder consumes significantly more slowly than polymerization inhibitors. In general, polymerization inhibitors are used to inhibit polymerization under normal process conditions; whereas, polymerization retarders are used to decrease polymerization reactions during an abnormal process condition, such as an emergency shutdown. The presence of both polymerization inhibitors and polymerization retarders in monomer production may be justified.
It is well known that undesirable and costly polymerization is a significant problem during the manufacturing of various vinyl monomers, particularly vinyl aromatic compounds (e.g., styrene, alpha-methylstyrene and vinyltoluene). Many kinds of polymerization inhibitors and polymerization retarders have been used in the past to minimize this problem. Examples of polymerization inhibitors that have been used to control polymer formation include alkyl-substituted di-nitro-phenols and nitrosophenols diethylhydroxylamine, phenyl-p-phenylenediamines, tert-butyl catechol, and phenothiazine. However, many of these compounds are difficult to handle, are expensive, and/or are regulated heavily with regards to their environmental effect.
Thus, it would be desirable if new polymerization retarders could be developed to inhibit and/or at least partially inhibit the rate of monomer polymerization and which are also cost effective.